The long term objective of this project is to study microembolization in patients with mechanical heart valves using transcranial Doppler (TCD) ultrasound monitoring. TCD detected microemboli are common in patients with mechanical heart valves. These patients are also at high risk for embolic stroke. Therefore this population is ideal for scientific study. We propose to study microemboli in both an in-vitro model as well as in patients with mechanical heart valves. The specific aim of the in-vitro project will be to determine the optimal ultrasound parameters for detecting microemboli. An artificial pulsatile flow model will be used in these studies. The primary aim of the clinical project is to identify the optimal length of TCD monitoring necessary to accurately determine the cerebral microembolization rate. Another aim of the clinical project is to assess whether microemboli respond to antithrombotic therapy. Patients will be monitored using a new multigated TCD monitoring system. Patients will be monitored for 8 hours/day over three consecutive days. During the second and third days either heparin or aspirin will be added to their therapy to determine if the rate of microembolization can be altered. After six months and 1 year the monitoring will be repeated to evaluate the variations in microembolization over time. If TCD monitoring is shown to be a determinate of stroke risk, this would have a major impact on the evaluation and treatment of hundreds of thousands of mechanical heart valve patients. It could potentially be used to identify patients stroke risk in other patient populations, potentially reducing the morbidity and mortality associated with the 500,000 new strokes that occur every year in the U.S.